Method for estimating the sulfur content in the fuel of an internal combustion engine

ABSTRACT

A method for estimating the sulfur content in the fuel of an internal combustion engine equipped with a catalyser; the percentage of sulfur present in the fuel supplied during a specific measurement time interval is estimated by dividing the quantity of sulfur stored in the catalyser during the measurement time interval by the product of a fixed conversion constant and the mass of fuel supplied to the cylinder in the measurement time interval.

[0001] The present invention relates to a method for estimating thesulfur content in the fuel of an internal combustion engine.

[0002] The present invention is advantageously applied in the automotiveinternal combustion engine sector, to which the following descriptionmakes explicit reference without thereby restricting the general scopethereof.

BACKGROUND OF THE INVENTION

[0003] Modern automotive internal combustion engines comprise an exhaustpipe that terminates in a catalyser, which has the function of reducinglevels of pollutants contained in the exhaust gas; in particular, thecatalyser stores either the NO_(x) groups produced during combustion, orthe sulfur (in the form of SO_(x)), which is contained in the fuel andis released during combustion. The catalyser has limited storagecapacity for NO_(x) groups and sulfur (such storage capacity generallyamounts to 3-5 grams) and when said storage capacity is exhausted, thecatalyser must be cleaned by means of a regeneration process.

[0004] The total mass of NO_(x) groups produced during combustion ismuch greater than the mass of sulfur released during combustion, andmoreover the regeneration process to remove NO_(x) groups (a few secondsof rich combustion) is much shorter than the regeneration process toremove sulfur (at least two minutes of rich combustion combined with aninternal temperature in the catalyser which, in relative terms, is veryhigh). For the reasons stated above, the regeneration process to removeNO_(x) groups is normally carried out every 45-75 seconds of engineoperation, while the regeneration process to remove sulfur is normallycarried out every 6-10 hours of engine operation.

[0005] In particular, the actual residual capacity available in thecatalyser for storing NO_(x) groups is estimated periodically accordingto the time elapsed since the preceding regeneration process to removesulfur and according to the sulfur content of the fuel, and performanceof the regeneration process to remove sulfur is scheduled on the basisof said estimate of residual capacity.

[0006] Fuel manufacturers guarantee the maximum sulfur content of fuel(for example in Italy said value is currently 150 ppm); however, theactual sulfur content is very often below said maximum value, such thatusing the maximum value results in an, often very significant,overestimate of sulfur content, so resulting in a greater frequency ofregeneration, which entails both increased consumption and greaterirregularity in engine operation. Moreover, the maximum sulfur contentin fuel varies from country to country, as a result of which an enginecalibrated to use a fuel in one country might not operate optimally withfuel from another country.

[0007] In order to resolve the problems described above, it has beenproposed to use a sensor capable of directly measuring the actual sulfurcontent of the fuel; however, said sensor is particularly expensive andnormally requires frequent calibration to provide accurate measurements.

SUMMARY OF THE INVENTION

[0008] The aim of the present invention is to provide a method forestimating the sulfur content in the fuel of an internal combustionengine, which method does not have the above-stated disadvantages and,in particular, is simple and economic to implement.

[0009] The present invention provides a method for estimating the sulfurcontent in the fuel of an internal combustion engine equipped with atleast one cylinder and at least one catalyser, the latter being capableof storing a quantity of sulfur and NO_(x) groups and being subjected toa regeneration process to remove NO_(x) groups when efficiency of thecatalyser itself falls outside an acceptable range; the method providinga determination of the percentage of sulfur present in the fuel suppliedduring a specific measurement time interval by dividing the quantity ofsulfur stored in the catalyser during the measurement time interval bythe product of a fixed conversion constant and the mass of fuel suppliedto the cylinder in the measurement time interval; the method beingcharacterised in that a first quantity of NO_(x) groups stored by thecatalyser immediately before a regeneration process to remove NO_(x)groups is estimated at the beginning of the measurement time interval, asecond quantity of NO_(x) groups stored by the catalyser immediatelybefore a regeneration process to remove NO_(x) groups is estimated atthe end of the measurement time interval and said quantity of sulfurstored in the catalyser during the measurement time interval isestimated from the difference between said first quantity of NO_(x)groups and said second quantity of NO_(x) groups.

[0010] The present invention also provides a method for estimating thequantity of sulfur stored in a catalyser of an internal combustionengine equipped with at least one cylinder; the catalyser being capableof storing a quantity of sulfur and NO_(x) groups and being subjected toa regeneration process to remove NO_(x) groups when efficiency of thecatalyser itself falls outside an acceptable range; the method beingcharacterised in that a first quantity of NO_(x) groups stored by thecatalyser immediately before a regeneration process to remove NO_(x)groups is estimated at the beginning of the measurement time interval, asecond quantity of NO_(x) groups stored by the catalyser immediatelybefore a regeneration process to remove NO_(x) groups is estimated atthe end of the measurement time interval and the quantity of sulfurstored in the catalyser during the measurement time interval isestimated from the difference between said first quantity of NO_(x)groups and said second quantity of NO_(x) groups.

[0011] The present invention also provides a method for estimating thesulfur content in the fuel of an internal combustion engine equippedwith at least one cylinder and at least one catalyser, the latter beingcapable of storing a quantity of sulfur and NO_(x) groups andperiodically being subjected to a regeneration process to remove sulfur;the method providing the use of a current sulfur concentration value andthe correction of said sulfur concentration value in order to obtain anew sulfur concentration value; the method being characterised in that afirst time interval, which is actually necessary to complete aregeneration process to remove sulfur, is measured, the quantity ofsulfur stored in the catalyser before said regeneration process toremove sulfur is determined using said current sulfur concentrationvalue, a second time interval, which is theoretically necessary tocomplete the regeneration process, is estimated on the basis of theestimated quantity of sulfur stored in the catalyser, and amultiplicative correction coefficient is determined as a ratio betweensaid first time interval and said second time interval.

BRIEF DESCRIPTION OF THE DRAWING

[0012] The present invention will now be described with reference to theattached drawing, which illustrates a non-limiting embodiment thereof;in particular, the attached FIGURE is a schematic diagram of an internalcombustion engine operating in accordance with the estimation methodprovided by the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0013] In the attached FIGURE, 1 denotes the overall internal combustionengine equipped with four cylinders 2 (only one of which is shown inFIG. 1), each of which is connected to an intake manifold 3 via at leastone respective intake valve 4 and to an exhaust manifold 5 via at leastone respective exhaust valve 6. The intake manifold 3 receives fresh air(i.e. air originating from the outside environment and containingapproximately 20% oxygen) via a throttle valve 7, which can be adjustedbetween a closed position and a maximally open position. The fuel (forexample petrol, diesel oil, methane or LPG) is directly injected intoeach cylinder 2 by a respective injector 8.

[0014] An exhaust pipe 9 leads from the exhaust manifold 5, said exhaustpipe comprising a precatalyser 10 and a subsequent catalyser 11; insidethe exhaust pipe 9 there is installed a UEGO probe 12, which is arrangedupstream from the catalytic preconverter 10 and is capable of detectingthe quantity of oxygen present in the exhaust gases input into thecatalytic preconverter 10, a temperature sensor 13, which is arrangedbetween the catalytic preconverter 10 and catalyser 11 and is capable ofdetecting the temperature of the gas input into the catalyser 11, and amultisensor 14, which is arranged downstream from the catalyser 11 andis capable of detecting either the presence of NO_(x) groups(nitrogenous group sensor) or the quantity of oxygen present relative tostoicheiometric conditions (lambda probe) in the exhaust gases outputfrom the catalyser 11 (i.e. in the exhaust gases released from theexhaust pipe 9 into the atmosphere).

[0015] The engine 1 furthermore comprises a control unit 15 which, interalia, on each cycle controls the throttle valve 7 and the injector 8 inorder to fill the cylinders 2 with a quantity of a blend of combustionagent (fresh air) and fuel in a specific ratio as a function of theoperating conditions of the engine 1 and as a function of the commandsreceived from the driver. In order to allow the control unit 15 toacquire the data required for correct operation thereof, the controlunit 15 is connected to the UEGO probe 12, the temperature sensor 13 andthe multisensor 14.

[0016] In service, the catalyser 11 stores either the NO_(x) groupsproduced during combustion or the sulfur (in the form of SO_(x))contained in the fuel and released during combustion in order to preventsaid constituents from being released directly into the atmosphere.Periodically, the control unit 15 calculates an index I of deteriorationin performance of the catalyser 11, which index I is capable ofindicating the efficiency with which the catalyser 11 itself isoperating.

[0017] The deterioration index I is stated as a percentage and iscalculated from the ratio between the quantity NO_(xloss) of NO_(x)groups not captured by the catalyser 11 and released directly into theatmosphere and the quantity NO_(xtotal) of NO_(x) groups produced by theengine 1; obviously, the higher the deterioration index I, the poorerthe performance of the catalyser 11. The quantity NO_(xloss) of NO_(x)groups not captured by the catalyser is obtained directly by the controlunit 15 by measurement, performed by the multisensor 14, of the exhaustgases released from the exhaust pipe 9 into the atmosphere, while thequantity NO_(xtotal) of NO_(x) groups produced by the engine 1 isobtained in substantially known manner by the control unit 15 using mapsthat state the specific quantity (i.e. the quantity per unit of fuelinjected into the cylinders 2) of NO_(x) groups produced by the engine 1as a function of engine status (typically as a function of engine speedand as a function of delivered torque).

[0018] The catalyser 11 has a limited storage capacity for NO_(x) groupsand sulfur (such storage capacity normally amounts to 4 grams) and whensaid storage capacity is exhausted, the catalyser 11 has to be cleanedby means of a regeneration process. The total mass of NO_(x) groupsproduced during combustion is much greater than the mass of sulfurreleased during combustion, and moreover the regeneration process toremove NO_(x) groups (a few seconds of rich combustion of the engine 1)is much shorter than the regeneration process to remove sulfur (at leasttwo minutes of rich combustion of the engine 1 combined with an internaltemperature in the catalyser 11 which, in relative terms, is very high).For the reasons stated above, the regeneration process to remove NO_(x)groups is normally carried out every 45-75 seconds of operation of theengine 1, while the regeneration process to remove sulfur is normallycarried out every 6-10 hours of operation of the engine 1.

[0019] In particular, the regeneration process to remove sulfur isscheduled by the control unit 15 according to the percentage value S ofsulfur contained in the fuel and according to the time that has elapsedsince the last regeneration process to remove sulfur, while theregeneration process to remove NO_(x) groups is carried out by thecontrol unit 15 every time the index I of deterioration in performanceof the catalyser 11 is greater than a preset threshold value (forexample 20%), since, under normal conditions, the deterioration index Itends to get worse (i.e. increase) as the storage capacity of thecatalyser 11 approaches saturation.

[0020] From the above explanation, it is clear that the total massMstored stored in the catalyser 11 is given by the sum of the quantitySO_(xstored) of stored sulfur, measured in NO_(x) equivalents, and ofthe quantity NO_(xstored) of stored NO_(x) groups, and that thecatalyser 11 is no longer capable of capturing further sulfur or NO_(x)groups, i.e. is no longer capable of operating properly, once the totalmass M_(stored) stored has come to equal the total storage capacity ofthe catalyser 11 itself.

[0021] The control unit 15 is equipped with an estimator 16, which iscapable of supplying the control unit 15 itself with an estimate of thepercentage S of sulfur present in the fuel used by the engine 1, so asto allow the control unit 15 to schedule correctly the regenerationprocesses for the catalyser 11 in order to achieve either reducedoverall consumption of the engine 1 or reduced emissions of pollutantsinto the atmosphere.

[0022] When the engine 1 is relatively new, i.e. when the catalyser 11is new and has not deteriorated, the estimator 16 is capable of directlyestimating the value of the percentage S of sulfur present in the fuelused by the engine 1; this function is of particular value for rapidlyobtaining a starting value for the percentage S of sulfur.

[0023] The percentage S of sulfur present in the fuel supplied during aspecific measurement time interval is estimated by the estimator 16 byapplying equation [1], in which SO_(xstored) is the quantity of sulfurstored in the catalyser 11 during the measurement time interval, K_(SOx)is a fixed conversion constant and mfuel is the mass of fuel supplied tothe cylinders 2 in the measurement time interval. $\begin{matrix}{S = \frac{{SOx}_{stored}}{K_{SOx} \cdot m_{fuel}}} & \lbrack 1\rbrack\end{matrix}$

[0024] The equation [1] is valid on the assumption that the sulfurcontained in the fuel is completely retained within the catalyser 11;this assumption substantially always applies, except for negligibleerrors during normal operation of the engine 1. Analysis of the equation[1] reveals that the value for the conversion constant K_(SOx) canreadily be determined theoretically and the value for the mass mfuel offuel supplied to the cylinders 2 in the measurement time interval can bedetermined easily and accurately by the control unit 15 on the basis ofthe commands issued to the injectors 8; it is thus clear that, once thevalue for the quantity SOxstored of sulfur stored in the catalyser 11has been estimated, the percentage S of sulfur can easily be calculated.

[0025] The quantity SO_(xstored) of sulfur stored in the catalyser 11 ina certain measurement time interval can be estimated by comparing theregeneration process to remove NO_(x) groups at the beginning of themeasurement time interval and the regeneration process to remove NO_(x)groups at the end of the measurement time interval and assuming that thedifference detected in the quantity of stored NO_(x) groups is entirelydue to the quantity SOxstored of sulfur stored in the catalyser 11; asstated above, this assumption is valid if the catalyser 11 has notdeteriorated and there is no drift in the model of the NO_(x) groups,i.e. when the catalyser 11 is substantially new.

[0026] In other words, it is assumed that, during the measurement timeinterval, the storage capacity of the catalyser 11 does not vary, i.e.it is assumed that the regeneration process to remove NO_(x) groups atthe beginning of the measurement time interval and the regenerationprocess to remove NO_(x) groups at the end of the measurement timeinterval proceed on the basis of the same value for total massM_(stored) stored in the catalyser 11. Since the total mass M_(stored)stored in the catalyser 11 is given by the sum of the quantitySO_(xstored) of stored sulfur, measured in NO_(x) equivalents, and ofthe quantity NO_(xstored) of stored NO_(x) groups, it is obvious thatthe difference found between the quantities NO_(xstored) of storedNO_(x) groups amounts to the quantity SO_(xstored) of stored sulfur.

[0027] The quantity NO_(xstored) of stored NO_(x) groups relating to theregeneration process to remove NO_(x) at the beginning of themeasurement time interval and relating to the regeneration process toremove NO_(x) groups at the end of the measurement time interval can beestimated by subtracting from the quantity NO_(xtotal) of NO_(x) groupsproduced by the engine 1 the quantity NO_(xloss) of NO_(x) groups notcaptured by the catalyser 11 and released directly into the atmosphere.As stated above, the quantity NO_(xloss) of NO_(x) groups not capturedby the catalyser is obtained directly by the control unit 15 bymeasurement, performed by the multisensor 14, of the exhaust gasesreleased from the exhaust pipe 9 into the atmosphere, while the quantityNO_(xtotal) of NO_(x) groups produced by the engine 1 is obtained in asubstantially known manner by the control unit 15 using maps that statethe specific quantity (i.e. the quantity per unit of fuel injected intothe cylinders 2) of NO_(x) groups produced by the engine 1 as a functionof engine status (typically as a function of engine speed and asfunction of delivered torque).

[0028] Under normal operating conditions, i.e. when the catalyser 11 isnot new, the estimator 16 is capable of adapting a current sulfurconcentration value SO_(old) by applying—where necessary—a correction tosaid current value S_(old) in order to obtain a new sulfur concentrationvalue S_(new).

[0029] The size of the above-stated correction to the current sulfurconcentration value S_(old) can be estimated during the regenerationprocess to remove sulfur, during which the engine 1 is caused to operatein rich combustion, by applying equation [2], in which t₀ is thestarting time for the regeneration process, t₁ is the measured real timeat which the multisensor 14 detects a change from lean (λ less than 1)to rich (λ greater than 1), and t₂ is the theoretical, estimated time atwhich the multisensor 14 ought to detect a change from lean (λ lessthan 1) to rich (λ greater than 1) if the current sulfur concentrationvalue S_(old) were correct. The value of time t₂ is easily calculated bycalculating the total quantity of sulfur present in the fuel injectedinto the cylinders 2 since the preceding regeneration process to removesulfur and assuming that said quantity of sulfur has been completelyretained by the catalyser 11; the total quantity of sulfur present inthe fuel is easily obtained by multiplying the total mass of fuelinjected by the current value S_(old) for sulfur concentration in thefuel. $\begin{matrix}{S_{new} = {S_{old} \cdot \frac{t_{1} - t_{0}}{t_{2} - t_{0}}}} & \lbrack 2\rbrack\end{matrix}$

[0030] During the regeneration process to remove sulfur, the multisensor14 detects lean operation (λ less than 1) for as long as sulfur ispresent in the catalyser 11, whereas it detects rich operation (λgreater than 1) when all the sulfur has been removed from the catalyser11; in other words the time interval (t₁−t₀) is a function of theassumed quantity of sulfur retained in the catalyser 11 and estimated bymeans of the current sulfur concentration value S_(old), while the timeinterval (t₂−t₀) is a function of the actual quantity of sulfur retainedin the catalyser 11.

[0031] From the above explanation, it is clear that the regenerationprocess to remove sulfur is not complete until the multisensor 14detects a change from lean (λ less than 1) to rich (λ greater than 1).

[0032] According to another embodiment, the size of the above-statedcorrection of the current sulfur concentration value Sold can beestimated by assuming that the dynamic sulfur filling process is fasterthan phenomena of drift in the engine 1 or of degradation of thecatalyser 11, i.e. by assuming that any difference D between anestimated value NO_(stored1) of the total quantity of stored NO_(x)groups by means of a model of NO_(x) group production by the engine 1and an estimated value NO_(xstored2) of the total quantity of storedNO_(x) groups on the basis of a storage model for the catalyser 11 isentirely attributable to an error in the current sulfur concentrationvalue Sold (current value S_(old) used in the storage model for thecatalyser 11).

[0033] In particular, if the difference D is less than a predeterminedthreshold, said difference is attributed to an error in the currentsulfur concentration value S_(old) and is used to correct the currentvalue S_(old), while if the difference D is greater than thepredetermined threshold, this indicates drift in the model of NO_(x)group production by the engine 1 and is used to adjust the model itself.

[0034] The estimated value NO_(xstored1) of the total quantity of storedNO_(x) groups is determined by using a model of NO_(x) group productionby the engine 1; in particular, use of such a model provides subtractionfrom the quantity NO_(xtotal) of NO_(x) groups produced by the engine 1of the quantity NO_(xloss) of NO_(x) groups not captured by thecatalyser 11 and released directly into the atmosphere. As stated above,the quantity NO_(xloss) of NO_(x) groups not captured by the catalyseris obtained directly by the control unit 15 by measurement, performed bythe multisensor 14, of the exhaust gases released from the exhaust pipe9 into the atmosphere, while the quantity NO_(xtotal) of NO_(x) groupsproduced by the engine 1 is obtained in a substantially known manner bythe control unit 15 using maps that state the specific quantity (i.e.the quantity per unit of fuel injected into the cylinders 2) of NO_(x)groups produced by the engine 1 as a function of engine status(typically as a function of engine speed and as function of deliveredtorque).

[0035] The estimated value NO_(xstored2) of the total quantity of storedNO_(x) groups is determined by using a model of storage by the catalyser11; said model is defined by a series of maps that state the quantity ofNO_(x) grou ps stored by the catalyser 11 as a function of the quantityNO_(xtotal) of NO_(x) groups produced by the engine 1 (obtained byapplying the above-described model of NO_(x) group production by theengine 1), as a function of the current sulfur concentration value Soldand as a function of the temperature of the gases present inside thecatalyser (temperature provided by the multisensor 14).

[0036] Obviously, the above-mentioned models, and in particular thevalues stored in the respective maps, are determined in the laboratoryby means of a series of tests carried out on the engine 1 equipped witha series of auxiliary measurement sensors, which are capable ofproviding an individual and accurate measurement of all the parametersinvolved in the operation of the engine 1 itself.

[0037] Preferably, the estimator 16 implements all three of the methodsdescribed above to estimate and/or correct the value S for sulfurconcentration in the fuel, so that it is possible to compare the resultsobtained with at least two different methods and to identify anyanomalous values due to malfunctioning or particular situations.

[0038] From the above explanation, it is clear that the estimator 16 iscapable of determining the current value S for sulfur concentration inthe fuel with a relatively high degree of precision; moreover,incorporating the estimator 16 inside the central control unit 15 isrelatively economical and simple in that it does not involve theintroduction of additional sensors, but simply modification at softwarelevel.

1. Method for estimating the sulfur content in the fuel of an internalcombustion engine (1) equipped with at least one cylinder (2) and atleast one catalyser (11), the latter being capable of storing a quantityof sulfur and NO_(x) groups and being subjected to a regenerationprocess to remove NO_(x) groups when efficiency (I) of the catalyser(11) itself falls outside an acceptable range; the method providing adetermination of the percentage (S) of sulfur present in the fuelsupplied during a specific measurement time interval by dividing thequantity (SO_(xstored)) of sulfur stored in the catalyser (11) duringthe measurement time interval by the product of a fixed conversionconstant (K_(SOx)) and the mass of fuel (m_(fuel)) supplied to thecylinder (2) in the measurement time interval; the method beingcharacterised in that a first quantity (NO_(xstored)) of NO_(x) groupsstored by the catalyser (11) immediately before a regeneration processto remove NO_(x) groups is estimated at the beginning of the measurementtime interval, a second quantity (NO_(xstored)) of NO_(x) groups storedby the catalyser (11) immediately before a regeneration process toremove NO_(x) groups is estimated at the end of the measurement timeinterval and said quantity (SO_(xstored)) of sulfur stored in thecatalyser (11) during the measurement time interval is estimated fromthe difference between said first quantity (NO_(xstored)) of NO_(x)groups and said second quantity (NO_(xstored)) of NO_(x) groups. 2.Method according to claim 1, in which each said quantity (NO_(xstored))of NO_(x) groups stored by the catalyser (11) is obtained by subtractingfrom a quantity (NO_(xtotal)) of NO_(x) groups produced by the engine(1) a quantity (NO_(xloss) ) of NO_(x) groups not captured by thecatalyser (11).
 3. Method according to claim 2, in which said quantity(NO_(xloss)) of NO_(x) groups not captured by the catalyser (11) ismeasured by a sensor located downstream from the catalyser (11). 4.Method according to claim 2, in which said quantity (NO_(xtotal)) ofNO_(x) groups produced by the engine (1) is estimated by using a modelof NO_(x) group production.
 5. Method according to claim 4, in whichsaid model of NO_(x) group production is defined by at least one mapthat states the specific quantity of NO_(x) groups produced by theengine (1) as a function of engine status.
 6. Method according to claim1, in which said efficiency (I) of the catalyser (11) is evaluated by anindex (I) of deterioration in the performance of the catalyser 11, whichindex (I) is calculated by means of the ratio between a quantity(NO_(xloss)) of NO_(x) groups not captured by the catalyser (11) and aquantity (NO_(xtotal)) of NO_(x) groups produced by the engine
 1. 7.Method for estimating the quantity (SO_(xstored)) of sulfur stored in acatalyser (11) of an internal combustion engine (1) equipped with atleast one cylinder (2); the catalyser being capable of storing aquantity of sulfur and NO_(x) groups and being subjected to aregeneration process to remove NO_(x) groups when efficiency (I) of thecatalyser (11) itself falls outside an acceptable range; the methodbeing characterised in that a first quantity (NO_(xstored)) of NO_(x)groups stored by the catalyser (11) immediately before a regenerationprocess to remove NO_(x) groups is estimated at the beginning of themeasurement time interval, a second quantity (NO_(xstored)) of NO_(x)groups stored by the catalyser (11) immediately before a regenerationprocess to remove NO_(x) groups is estimated at the end of themeasurement time interval and said quantity (SO_(xstored)) of sulfurstored in the catalyser (11) during the measurement time interval isestimated from the difference between said first quantity (NO_(xstored))of NO_(x) groups and said second quantity (NO_(xstored)) of NO_(x)groups.
 8. Method according to claim 7, in which each said quantity(NO_(xstored)) of NO_(x) groups stored by the catalyser (11) is obtainedby subtracting from a quantity (NO_(xtotal)) of NO_(x) groups producedby the engine (1) a quantity (NO_(xloss)) of NO_(x) groups not capturedby the catalyser (11).
 9. Method according to claim 8, in which saidquantity (NO_(xloss)) of NO_(x) groups not captured by the catalyser(11) is measured by a sensor (14) arranged downstream from the catalyser(11).
 10. Method according to claim 8, in which said quantity(NO_(xtotal)) of NO_(x) groups produced by the engine (1) is estimatedby using a model of NO_(x) group production.
 11. Method according toclaim 10, in which said model of NO_(x) group production is defined byat least one map that states the specific quantity of NO_(x) groupsproduced by the engine (1) as a function of engine status.
 12. Methodaccording to claim 7, in which said efficiency (I) of the catalyser (11)is evaluated by an index (I) of deterioration in the performance of thecatalyser 11, which index (I) is calculated by means of the ratiobetween a quantity (NO_(xloss)) of NO_(x) groups not captured by thecatalyser (11) and a quantity (NO_(xtotal)) of NO_(x) groups produced bythe engine
 1. 13. Method for estimating the sulfur content in the fuelof an internal combustion engine (1) equipped with at least one cylinder(2) and at least one catalyser (11), the latter being capable of storinga quantity of sulfur and NO_(x) groups and being subjected to aregeneration process to remove sulfur; the method providing the use of acurrent sulfur concentration value (S_(old)) and the correction of saidcurrent sulfur concentration value (S_(old)) in order to obtain a newsulfur concentration value (S_(new)); the method being characterised inthat a first time interval (t₁−t₀), which is actually necessary in orderto complete a regeneration process to remove sulfur, is measured, thequantity (SO_(xstored)) of sulfur stored in the catalyser (11) beforesaid regeneration process to remove sulfur is determined using saidcurrent sulfur concentration value (SO_(old)), a second time interval(t₂−t₀), which is theoretically necessary to complete the regenerationprocess, is estimated on the basis of the estimated quantity(SO_(xstored)) of sulfur stored in the catalyser (11), and amultiplicative correction coefficient is determined as a ratio betweensaid first time interval (t₁−t₀) and said second time interval (t₁−t₀).14. Method according to claim 13, in which the end of said first timeinterval (t₁−t₀), which is actually necessary to complete theregeneration process to remove sulfur is determined according to thetime (t₁) at which a lambda probe (14) arranged downstream from thecatalyser (11) detects a change from lean to rich in the gases emittedby the catalyser (11) itself.
 15. Method according to claim 13, in whichthe quantity (SO_(xstored)) of sulfur stored in the catalyser (11) isestimated by multiplying the current sulfur concentration value(S_(old)) by the total mass (m_(fuel)) of fuel supplied to the cylinder(2) since the preceding regeneration process to remove sulfur. 16.Method according to claim 14, in which the quantity (SO_(xstored)) ofsulfur stored in the catalyser (11) is estimated by multiplying thecurrent sulfur concentration value (S_(old)) by the total mass(m_(fuel)) of fuel supplied to the cylinder (2) since the precedingregeneration process to remove sulfur.
 17. Method for estimating thesulfur content in the fuel of an internal combustion engine (1) equippedwith at least one cylinder (2) and at least one catalyser (11), thelatter being capable of storing a quantity of sulfur and NO_(x) groups;the method providing the use of a current sulfur concentration value(S_(old)) and the correction of said current sulfur concentration value(S_(old)) in order to obtain a new sulfur concentration value (S_(new));the method being characterised in that a first value (NO_(xstored1)) ofthe total quantity of NO_(x) groups stored in the catalyser (11) isestimated by means of a model of NO_(x) group production by the engine(1), a second value (NO_(xstored2)) of the total quantity of NO_(x)groups stored by the catalyser (11) is estimated by means of a model ofstorage of the catalyser (11) and an additive correction coefficient (D)is determined as the difference between said first estimated value(NO_(xstored1)) and said second estimated value (NO_(xstored2)). 18.Method according to claim 17, in which said correction coefficient (D)is used to correct said current sulfur concentration value (S_(old)) ifthe correction coefficient (D) is within a predetermined range, while itis used to adjust the model of NO_(x) group production by the engine (1)if the correction coefficient (D) is outside the predetermined range.19. Method according to claim 17, in which said model of storage by thecatalyser (11) is defined by a series of maps that state the quantity ofNO_(x) groups stored by the catalyser (11) as a function of the quantity(NO_(xtotal)) of NO_(x) groups produced by the engine (1), as a functionof the current sulfur concentration value (S_(old)) and as a function ofthe temperature of the gases present inside the catalyser (11). 20.Method according to claim 17, in which said model of NO_(x) groupproduction by the engine (1) provides obtaining the quantity(NO_(xstored)) of NO_(x) groups stored by the catalyser (11) bysubtracting from a quantity (NO_(xtotal)) of NO_(x) groups produced bythe engine (1) a quantity (NO_(xloss)) of NO_(x) groups not captured bythe catalyser (11).
 21. Method according to claim 20, in which saidquantity (NO_(xloss)) of NO_(x) groups not captured by the catalyser(11) is measured by a sensor located downstream from the catalyser (11).22. Method according to claim 20, in which said quantity (NO_(xtotal))of NO_(x) groups produced by the engine (1) is estimated by using amodel of NO_(x) group production.
 23. Method according to claim 22, inwhich said model of NO_(x) group production is defined by at least onemap that states the specific quantity of NO_(x) groups produced by theengine (1) as a function of engine status.